The present invention relates to a rapid temperature sensor made of metal of the platinum group for high-temperature sensor technology, in particular for use in motor-vehicle exhaust-gas monitoring technology.
Many applications in the field of high-temperature sensor technology (for example, emission-controlled combustion) require sensors which can detect the temperatures in their environment with time constants of less than 100 ms. In the field of exhaust-gas sensor technology it is, for example, necessary to keep the gas-sensitive elements at a constant temperature of approximately 1,000.degree. C. The exhaust-gas temperature variations must be detected as rapidly as possible in order to be able to control the heating elements integrated on the sensor chip in such a way that the temperature of the gas-sensitive layer remains virtually constant.
For application temperatures of up to 1,000.degree. C., platinum/platinum-rhodium or nickel/chromium-nickel thermocouples have hitherto been used. These couples have, however, two crucial disadvantages. On the one hand, they cannot be integrated on hybrid components and, on the other hand, they have relatively long response times to temperature changes in their environment. Temperature sensors which are made of metals of the platinum group and which have been screen-printed or produced using other thin-film technologies (PVD or CVD processes) have hitherto been available for use only up to a maximum of 850.degree. C. since, starting from this temperature as a maximum, the platinum metals form both stable and volatile oxides which result in a resistance drift of the temperature sensor.
To protect the temperature sensor made of metal of the platinum group against oxidation and consequently prevent the resistance drift of the sensor, the metal of the platinum group is covered with various protective layers. For example, GB 2 171 253 discloses a temperature sensor which is made of platinum metal and provided with an aluminum oxide protective layer.
JP-A 63269502 discloses a platinum resistance film having a silicon nitride protective layer.
A platinum layer resistance having a titanium dioxide/silicon dioxide double layer is to be found in German Offenlegungsschrift 36 03 757.
An article by J. C. Godefreuy et al. in J. Vac. Sci. Technol. A 5 (5), 1987, pages 2917 to 2923 discloses a platinum thin-film temperature sensor which has a complicated multilayer structure comprising a platinum thin film embedded in aluminum oxide.
A disadvantage of the known sensors is that, despite passivation layers and protective layers, an oxidation of the platinum which results in the unacceptable drift of the temperature sensor is observed at temperatures of over 850.degree. C. The oxidation can be attributed, in particular, to cracks in the passivation layers, which cracks develop during cooling from high temperatures down to room temperature because of the different coefficients of thermal expansion of the materials used for the sensor.
During cooling, glass layers to be applied as passivation layer using thick-film technology also form cracks which permit access of oxygen and consequently an oxidation of the platinum.